This invention relates to drugs such as spironolactone which block the activity of the hormone aldosterone, and to the use of aldosterone-blocking drugs to prevent or treat myocardial fibrosis, a disease condition.
In a medical context, fibrosis refers the creation of fibrotic tissue (i.e., tissue characterized by an abnormally high quantity of fibrous material, primarily strands of collagen). In some situations, fibrosis is useful and necessary, such as in the healing of wounds, but in other situations, fibrosis can be harmful, especially when it interferes with the functioning of internal organs. As one example, liver cirrhosis is usually characterized by high levels of fibrosis. That condition, discussed in the-above-cited parent application Ser. No. 07/871,390, is not directly relevant to this invention.
This invention relates to the use of mineralocorticoid antagonists (such as spironolactone) in inhibiting myocardial fibrosis.
The correlation between mineralocorticoids and fibrosis was not recognized prior to the work of the Applicant. However, a great deal was known about mineralocorticoids and about fibrosis, as separate fields in medicine and physiology. Accordingly, the following sections provide background information on each of those topics.
Mineralocorticoids
The adrenal glands, which sit on top of the kidneys in the human body, are divided into two portions: the adrenal medulla (which secretes epinephrine and norepinephrine), and the adrenal cortex. The adrenal cortex secretes a number of hormones known as corticoids, which are divided into two categories. Glucocorticoids (primarily hydrocortisone, also known as cortisol) exert their primary effects on the metabolism of glucose and other carbohydrates; they can also secondarily retard wound healing, by interfering with inflammatory cell and fibrous tissue responses. The primary effects of mineralocorticoids (MC's) involve the retention of certain minerals, particularly sodium, and the elimination of potassium.
The most important and potent MC is aldosterone (ALDO); another naturally occurring MC which is less potent is deoxycorticosterone (DOC). If ALDO is present at abnormally high quantities (such as following hemorrhage, bodily injury, or sodium deprivation), the body will retain sodium and water, and will secrete potassium. This can be a beneficial short-term response to stress. However, chronic elevations of ALDO can be detrimental, such as in a patien: with heart failure who is suffering from edema (fluid accumulation), or a patient with hypertension (high blood pressure). In patients with edema or hypertension, an excess of ALDO promotes salt and water retention and potassium loss, which are detrimental. Certain drugs, most notably spironolactone and epoxymexrenone (discussed below), can be used to suppress activity of elevated circulating ALDO, or to suppress the synthesis of ALDO.
ALDO secretion is influenced by various signals involving adrenocorticotropin hormone (ACTH), melanocyte stimulating hormone, atrial natriuretic peptide, and plasma concentrations of sodium and potassium, and by a multi-step pathway called the renin-angiotensin-aldosterone (RAA) system. In response to certain signals which indicate a low blood pressure, the kidneys secrete renin, which cleaves a precursor peptide called angiotensinogen to release a peptide having ten amino acid residues, called angiotensin I. This peptide is cleaved by another enzyme called angiotensin converting enzyme (ACE) to generate angiotensin II, which has eight amino acid residues. In addition to being a potent vasoconstrictor (which increases blood pressure), angiotensin II functions as a hormone to stimulate the release of ALDO by zona glomerulose cells in the adrenal cortex. The RAA system is described in more detail in articles such as Weber and Brilla 1991 (full citations are provided below).
ALDO receptors (also called mineralocorticoid receptors (MCR or MinR, or mineralosteroid receptors) are proteins that initially reside in the cytoplasm of certain types of cells, such as smooth muscle cells and fibroblasts in the aorta (see, e.g., Meyer and Nichols 1981). When an ALDO receptor is activated by ALDO, the receptor/ALDO complex (or lease some portion thereof) is transported into the cell nucleus, where it binds to nuclear chromatin and presumably causes an alteration in the transcription of genes that encode proteins which are involved in the retention of sodium and water by the body (see, e.g., Kornel et al. 1983). For more information on ALDO receptors, see Agarwal and Lazar 1991 and additional references cited therein. Chemical methods of synthesizing aldosterone are described in articles such as Barton et al. 1975 and Miyano 1981.
Fibrosis
Fibrosis (the generation of fibrotic tissue) is important in a number of processes in adult mammals. In some processes, such as wound healing, fibrosis is highly beneficial and essential to survival. When one or more blood vessels, which function as barriers to separate the intravascular and extracellular spaces, are cut or otherwise broken or disrupted, an orderly wound healing process is initiated. Certain types of blood cells such as platelets release fibrinogen, plasminogen, and fibronectin into the cellular interstitium; these molecules react with other molecules to generate an extravascular coagulation and the formation of a hydrophilic fibrin-fibronectin gel. Various growth factors are believed to orchestrate the subsequent entry of immune and inflammatory cells and fibroblasts into the gel and the formation of new blood vessels within its interstices.
During the early stages, the gel is considered granulomatous tissue. It is gradually absorbed and replaced by fibrous tissue. One of the important components of such tissue is collagen, a fibrous protein secreted by fibroblasts; it provides an intercellular lattice or matrix which anchors cells in position in cohesive tissue (such as muscle or blood vessels).
In addition to synthesizing and secreting collagen, fibroblasts also synthesize and secrete collagenase, an enzyme which digests collagen. In healthy tissue, the gradual cycle of collagen secretion and degradation helps ensure that the protein fibers and connective tissue remain flexible, elastic, and in a steady-state concentration.
If fibrosis occurs as a wound-healing process in response to injury, it is classified as "reparative" fibrosis. Similarly, if fibrosis is initiated in an internal organ in response to the necrosis of parenchymal cells (i.e., cells which are characteristic of that particular organ, as distinct from non-specific cells), the generation of fibrotic tissue constitutes reparative fibrosis. In either case, collagen accumulation and connective tissue formation usually resemble scar tissue.
In some situations which can generally be regarded as disease conditions, blood vessels can lose their integrity and become permeable to macromolecules, even in the absence of a cut or other injury. In such situations, fibrosis can arise which is unwanted and unnecessary; it resembles a wound healing response that has gone awry. In the absence of parenchymal cell loss, this type of fibrosis can be classified as a "reactive" fibrosis. If a sufficient quantity of unwanted fibrotic tissue is generated in an internal organ such as the heart, the fibrotic tissue can compromise or seriously damage the functioning of the organ.
Humans or animals that suffer from chronic hypertension or edema often are found, upon autopsy, to have heart muscle that suffers from a characteristic often referred to by pathologists as "tough beef." Instead of appearing pliable, elastic, and free of stranded material, like a fresh high-quality filet mignon, the heart muscle is riddled and interspersed with fibrotic strands which render the heart muscle stiff and unable to flex, move, and function with full efficiency. This condition is referred to as myocardial fibrosis when it involves heart muscle (in medicine, the prefix "myo" refers to muscle), or as cardiac fibrosis (which is somewhat broader, since it can also include fibrosis in coronary arteries).
Myocardial fibrosis can be generated in lab animals in any of several ways. Animals models which have been reported in the literature (e.g., Doering et al. 1988 and Brilla et al. 1990) involve: (1) renovascular hypertension (RHT), which can be induced by surgically placing a constricting band around a renal artery for a prolonged period (such as several weeks, in rats); and, (2) ALDO-induced hypertension, in which animals are administered ALDO (usually be means of a small osmotic pump implanted beneath the skin, which slowly releases ALDO over a period of weeks) while being fed a high-salt diet. Either of these interventions, discussed in more detail below, will provoke an unwanted myocardial fibrosis in a previously normal heart.
One of the main forms of myocardial fibrosis involves a condition known as "left ventricular hypertrophy," which is discussed at some length in Weber and Brilla 1991. The left ventricle is the largest pumping chamber of the heart; it pumps blood through the entire body and head, excluding the lungs (which receive blood from the right ventricle). "Hypertrophy" refers to a non-tumorous increase in the size of an organ or muscle, and in left ventricular hypertrophy (LVH), the muscular wall of the left ventricle becomes excessively large. LVH is the single most important risk factor associated with adverse myocardial events, including myocardial failure and sudden death.
In animal models, LVH can be induced (for the purpose of studying it) by various means that generate hypertension over a prolonged period of time. Such interventions include (1) surgically constricting an artery that supplies the kidneys; (2) feeding animals a high-sodium diet coupled with ALDO injections; and, (3) infrarenal aortic banding, which involves surgically clamping the abdominal aorta below the junction where the renal arteries branch off; this reduces blood flow to the legs and elevates blood pressure in the thoracic region. In many but not all cases, LVH is accompanies by fibrosis (see, e.g., Jalil et al. 1988 and 1989).
Fibrosis has also been observed as a result of steroid use or abuse by humans, and steroid administration to lab animals; see, e.g., Skelton 1954, Hassager et al. 1990, and Luke et al.
Most types of reactive fibrosis are initially characterized by "perivascular" fibrosis (i.e., fibrosis which is localized around blood vessels. As the process of fibrosis continues and fills the spaces between various types of cells, it can be characterized as an "interstitial" fibrosis.
Fibrosis is intimately related to collagen metabolism by fibroblast cells. In the repair of wounds or cell necrosis, the activity of fibroblasts in synthesizing fibrillar collagen and in secreting collagenase will have a major impact on whether the fibrous tissue component of the wound healing or tissue replacement response is appropriate and proceeds to a successful conclusion. In reactive fibrosis, the amount of undesired collagen deposition will also depend on the level of activity of the fibroblasts involved.
Various factors are known to regulate collagen metabolism and fibroblast growth, or both, and thereby govern collagen accumulation (see, e.g., Rothe and Falanga 1989). These include cytokines such as fibroblast growth factor, platelet derived growth factor and transforming growth factor-beta.sub.1. Hormones are also involved, such as the above-mentioned glucocorticoids, which oppose various aspects of wound healing including inflammatory cell and fibrous tissue responses and which have antifibrotic properties relative to wound healing.
The processes of myocardial fibrosis, and the structure and arrangement of collagen fibers and cells in heart muscle, are described and illustrated in articles such as Weber and Brilla 1991. Articles which focus more specifically on the role of cytokines and other growth factors on wound healing include Blitstein-Willinger 1991 and Rothe and Falanga 1989.
Anti-Aldosterone Drugs
A number of drugs have been identified which can inhibit the activity of ALDO in the body, including spirolactones. The term "spirolactone" indicates that a lactone ring (i.e., a cyclic ester) is attached to another ring structure in a spiro configuration (i.e., the lactone ring shares a single carbon atom with the other ring). Spirolactones which are coupled to steroids are the most important class of spirolactones from a pharmaceutical perspective, so they are widely referred to in the pharmaceutical arts simply as spirolactones. As used herein, "spirolactone" refers to a molecule comprising a lactone structure coupled via a spiro configuration to a steroid structure or steroid derivative.
One particular spirolactone which functions as an effective ALDO antagonist is called bpironolactone, which is marketed as an anti-hypertensive and diuretic drug by G. D. Searle (Skokie, Ill.) under the trademarks "Aldoctone" and "Aldactazide." Spironolactone is the name commonly used by chemists; the full chemical name is 17-hydroxy-7-alpha-mercapto-3-oxo-17-alpha-pregn-4-ene-21-carboxylic acid gamma-lactone acetate. This compound, its activities, and modes of synthesis and purification are described in a number of U.S. patents, including U.S. Pat. No. 3,013,012 (Cella and Tweit 1961) and U.S. Pat. No. 4,529,811 (Hill and Erickson 1985). Another spironolactone effective as an aldosterone antagonist is epoxymexrenone, which has the structural formula set forth below: ##STR1##
Epoxymexrenone possesses high mineralocorticoid receptor affinity (comparable to spironolactone) but with reduced binding affinity for androgen and progesterone receptors. Initial studies of this compound have demonstrated a Na.sup.+ /K.sup.+ effect equipotent with spironolactone at a 50 mg dose.
Spironolactone and epoxymexrenone function as antagonists of ALDO; they occupy ALDO receptors without triggering the normal receptor activity. This competitive binding reaction reduces the ability of ALDO molecules to bind to and trigger activity at such receptors. As used herein, "aldosterone antagonist" refers to any compound that suppresses the receptor-mediated activity of aldosterone; it does not include compounds which reduce the amount of aldosterone synthesized or secreted by the adrenal cortex, such as mespirenone (discussed below).
When spironolactone and epoxymexrenone are used to suppress ALDO activity, they promote the elimination of fluid and sodium by the body, primarily via the kidneys and its formation of urine. Both of these effects help control hypertension in people suffering from high blood pressure. Spironolactone is therefore used to treat hypertension, and epoxymexrenone has been demonstrated as effective for this same purpose. The minimum effective anti-hypertensive dosage of these spirolactone compounds in adults is about 50 milligrams (mg) per day; dosages often exceed this, and dosages of 200 to 400 mg/day are common for chronic treatment. Since spironolactone and epoxymexrenone are metabolized and secreted fairly rapidly, typical administration involves pills containing 25 to 100 mg, taken four times daily.
The anti-hypertensive dosage of aldosterone antagonists is important to the subject invention, because it has been discovered that spirolactones such as spironolactone and epoxymexrenone can be used for an entirely different purpose (to inhibit fibrosis, as described herein) at dosages that are below the dosages which have anti-hypertensive effects. This is a useful finding, since it indicates that spirolactones and other aldosterone antagonists can be used to prevent unwanted fibrosis at dosages which have minimal side effects and do not substantially alter the body fluids or mineral concentrations of a patient.
As mentioned above, ALDO is secreted as one step in a multi-step pathway involving renin and angiotensin, in the RAA system. Various drugs have been identified which can inhibit one or more of the steps in this pathway. For example, if a drug classified as an "ACE inhibitor" (i.e., it inhibits angiotensin converting enzyme) is used to suppress the formation of angiotensin II, secretion of ALDO by the adrenal cortex will be suppressed. The most widely used ACE inhibitor is captopril.
In addition, certain drugs have been identified which appear to block ALDO synthesis and/or secretion by a more direct mechanism. These drugs include 15,16-methylene spirolactone compounds called Mespirenone (also called ZK 94679) and dethiolated Mespirenone (also called ZK 91587); see Losert et al. 1986, Nickisch et al. 1991, and Agarwal and Lazar 1991).
One object of this invention is to disclose a method of using an aldosterone antagonist, at a dosage which does not disrupt a patient's normal electrolyte and water-retention balance, to treat or prevent myocardial fibrosis. This and other objects will become more apparent in the following summary and description.